1) Field of the Invention
The present invention relates to controlling an operation of an internal combustion engine that includes a port injection valve and a direct injection valve.
2) Description of the Related Art
In a direct-injection internal combustion engine, fuel is directly injected into the cylinder. The direct-injection internal combustion engine can operate in a stratified combustion and a uniform combustion.
In the stratified combustion, fuel is injected in the cylinder during a compression stroke and a stratification of fuel is formed in the cylinder. Precisely, a mixture of fuel and air that is easy to ignite is accumulated near the spark plug, and the air that is hard to ignite is made to surround the mixture. The stratified combustion can produce ultra lean combustion. In other words, the stratified combustion allows both the reduction in the amount of the fuel and the reduction in the CO2 emission.
On the other hand, in the uniform combustion, fuel is injected in the cylinder during an intake stroke, and the fuel is made to disperse uniformly inside the cylinder. In the uniform combustion, intake air can be cooled by the heat of vaporization of the fuel, which allows better filling efficiency and, therefore, higher output. Therefore, the engine is operated in the uniform combustion if high torque is required.
In the uniform combustion, a large amount of fuel is injected in the cylinder particularly at the time of high output or high load. However, if a large amount of fuel is injected in the cylinder at one time, the fuel does not evaporate effectively. This causes improper combustion and results in a decrease in the torque. Japanese Patent Application Laid-Open Publication No. 2001-20837 discloses a solution to this problem. The engine disclosed in this publication includes a main fuel injection valve that injects fuel directly into a cylinder and an auxiliary fuel injection valve that injects fuel into an intake port. Moreover, how much fuel is to be injected from both the main fuel injection valve and the auxiliary fuel injection valve is controlled based on an operating state of the engine.
When the uniform combustion is employed, the fuel is injected from the direct injection valve during an intake stroke. However, knocking easily occurs if the internal combustion engine is operated at a high load and at low to medium speeds. Therefore, conventionally, torque of the internal combustion engine is sacrificed to suppress the knocking.